243 (#315) ############################################ PRINCIPLES AND PRACTICE > heart-wheel action (Fig. 72A, p. 249) will bind in the beard of the bobbin, but will not result in a smarter yarn being spun. This gain, however, is usually well worth considering. It will be noted that the spindle speed in all spinning frames is the limit of production, the delivery of the front rollers being controlled to give the required turns per inch of twist. This necessitates the back-rollers being driven from the front rollers or else every modifica- tion in the twist would be accompanied by a modifica- tion in the draft. From the plan of the flyer frame given in Fig. 69 the necessary changing for draft will be realized, and also the conditions of regulating the twist. The cap spindle (Fig. 68b) is in a manner the reverse of the flyer, the bobbin being driven and winding the the yarn--as delivered from the front rollers-upon itself by the action of the air friction and possibly the friction of the yarn on edge of the cap. Thus in this type of frame the spindles must be driven at a high speed (at least 4,000 revolutions per minute and up to 7,000), otherwise the yarn will be softly and insecurely wound on to the spool. This is probably the most "natural" method of all the spinning methods. It is curious to note that while cap spinning has become the method of spinning par excellence for fine Botany and cross-bred yarns it has never proved successful for cotton yarns. This fact again suggests how little is really known about spinning principles, It is obvious that the shape of the cap will be varied to meet two exigencies, viz. : shape of bobbin ; delicacy of thread frictioning. The double-headed bobbin will require a cap shaped 243